Technical Field
The present disclosure relates to radiation detectors, such as radiation detectors used in computed tomography (CT). More particularly, the present disclosure relates to a direct conversion radiation detector and detection method utilizing a cathode with an uneven surface for focusing field lines of an electric field at particular locations along the uneven surface of the cathode to reduce polarization.
Description of Related Art
A medical imaging system may include an array of indirect conversion (scintillator/photosensor) detectors such as a gadolinium oxysulfide (GOS) detectors or direct conversation detectors such as Cadmium Zinc Telluride (CZT) detectors. Direct conversion detectors directly convert detected radiation to electrical signals such as an electrical current.
An example direct conversion detector includes a CZT radiation sensitive semiconductor substrate with a first side that receives radiation and a second opposing side with electrical contacts for transferring corresponding electrical signal. The radiation sensitive semiconductor substrate may be partitioned into a plurality of rows of detector elements and a plurality of columns of detector elements to form a two dimensional array of detector elements. Each detector element is associated with a corresponding electrical contact for transferring the corresponding electrical signal to a readout substrate, which in turn includes electrical contacts for transferring the electrical signal off of the detector.
Radiation detectors, e.g., detectors capable of detecting X-rays and/or gamma rays, have been developed over the years for a variety of applications, e.g., medical imaging and detection, non-destructive testing and security inspection. Some early detectors included a collimator, a scintillation crystal and a plurality of photomultiplier tubes (PMTS). To overcome some shortcomings associated with PMT detectors, direct conversion detectors have been developed. Direct conversion detectors are capable of operating in photon counting mode or current mode.
Direct conversion radiation detectors, e.g., radiation detectors using Cadmium Zinc Telluride (CZT) or some other direct conversion material, have been developed for a variety of applications. Research on CZT has been primarily devoted to photon counting applications. Typically, these photon counting applications use gamma sources with flux rates that range from 1 photon per second up to at least 106 photons per second. These high flux rate applications may use Bremstrahlung sources, such as conventional X-ray tubes. These sources typically supply much higher fluence than gamma sources, even at their lowest range of operation, which is in the range of 106 photons per second to up to 109 photons per second. What's more is that X-ray tubes are polychromatic sources that output a wide spectrum of energies which has a significant effect on how these photons interact with the detection material.
Direct conversion radiation detectors traditionally have been plagued by polarization effects for high count rates in photon counting mode and non-planar response in current mode. The cause of the polarization may be the result of one of the carriers, either electron or hole, having a significantly lower mobility(μ)-lifetime(τ) product (μτe—mu-tau electrons; μτj—mu-tau holes) than the other carrier.
Therefore, there is an increasing need to develop radiation-sensitive detection filters that minimize polarization effects.